The agent was responding to a report of illegal activity near a culvert and discovered a group of undocumented immigrants, the agency said. The agent tried to apprehend the group, but he came under attack by people using blunt objects. The agent fired at least one round, fatally wounding one person, CBP said.

The woman was shot in the head, said Fire Chief Juan C. Gonzalez of Rio Bravo Fire & Rescue. She still had a pulse when the department arrived at the scene at 12:47 p.m., he said. But by the time his team got supplies, she no longer had a pulse, Gonzalez said.

Members of the department performed CPR for eight minutes before EMTs arrived and pronounced her dead, Gonzalez said.

‘You Killed Her’

Rio Bravo is on the border with Mexico, about 170 miles south of San Antonio.

In a bystander video posted on Facebook, a woman yells at an agent: “Why are you mistreating them? Why are you mistreating them? Why did you (shoot) at the girl? You killed her. He killed the girl. She’s laying there and she’s dead.”

The video showed an agent leading away some men. CBP said responding agents apprehended three undocumented immigrants.

The FBI and Texas Rangers are investigating. FBI agents are expected to interview the three people taken into custody, the law enforcement source said Thursday. The names of the agent and the deceased have not been released.

A law enforcement source said the immigrant was female. Webb County Medical Examiner Corinne Stern said her office has been in touch with the Guatemalan consulate as they work to identify her.

The Texas branch of the American Civil Liberties Union called for all border agents to begin wearing body cameras, for the sake of transparency.

“While we do not yet have all the facts in this case, Border Patrol’s history of violence against immigrants requires us to scrutinize every incident involving lethal force closely,” said Astrid Dominguez, director of the group’s Border Rights Center.

“We call on the Texas Rangers and the FBI to conduct their investigation thoroughly and transparently, and we demand that Border Patrol expand its use of body cameras to include each and every one of its agents in the field,” she said.

The Dangers For Border Patrol Agents

The incident comes as lawmakers continue to say that Border Patrol agents face rising dangers on the job and more than a month after President Donald Trump ordered the deployment of National Guard troops to secure the US-Mexico border.

But Border Patrol agents lead far safer work lives on average than most other law enforcement officers. Data show that assaults against the 19,500 agents of the Border Patrol have been dropping, not rising.

After surging sharply in fiscal year 2017, according to CBP figures, assaults against agents have fallen 32% through the end of March, compared with last year.

Border Patrol agents have reported being assaulted at less than half the rate of other law enforcement officers.

In fiscal 2016, the most recent year for which figures are available, the nearly 600,000 law enforcement officers whose agencies reported assaults and deaths to the FBI suffered one assault per 10 officers, compared with one assault for every 44 Border Patrol agents.

Gonzalez, chief of Rio Bravo Fire & Rescue, says that he and colleagues have been treated aggressively by undocumented immigrants passing through this area.

The CBP officer involved in the incident is being checked for injuries, Gonzalez said.